Automatic level control



March 28, 1944. BOYKlN AUTOMATIC LEVEL CONTROL Filed llay 2, 1942' INVENTOB Ja/m f8 Boy/M7.

V'WITNESSES: m v

ATTORNEY Patented Mar. 1944 AUTOMATIC LEVEL CONTROL John R. Boykin, Baltimore, MIL, assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 2, 1942, Serial No. 441,567

Claims.

This invention relates to control circuits, and more particularly, to circuits for preventing excessive voltage magnitudes in an electrical transmission system.

With the trend to high level modulation and higher power transmitters, there is an ever increasing need for a circuit arrangement to prevent high potential surges appearing on the program line of a radio transmitter which would overload the modulation circuit.

An objectof this invention is to eliminate the deleterious effect of voltage amplitudes ina transmission circuit exceeding a certain predetermined level and to this end, means are provided for controlling the effective impedance of variable impedance elements shunting said circuit inversely in accordance with the increase of voltage amplitudes. I

Another object of this invention is to prevent voltage surges appearing due to unbalance in a balanced transmission circuit which would seriously affect the transmission network. To this end means are provided to control the effective impedance of variable impedance elements in accordance with voltage magnitudes derived fromcurrents produced due to unbalance in the transmission circuit.

A particular feature of the invention is that the variable impedance elements utilized in the transmission circuit comprise vacuum tubes which derive operating potentials from the signal voltage in the transmission circuit and control potentials from the currents produced by unbalance in the transmission line feeding the circuit.

A particular advantage of the control system in accordance with this. invention is that the vacuum tubes as impedance elements have in finite impedance at normal operating potentials, whereas upon conditions of unbalance the impedance thereof decreases sharply as the unbalance exceeds a predetermined magnitude.

Other features and'advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims, and taken in connection with the accompanying drawing, in which:

Figure 1 is a schematic circuit arrangement of a balanced transmission system and a control circuit therefor; and

Figs. 2 and 3 show the signal voltage fluctua tions in the transmission system with a sudden peak surge and the operation of the control circuit in eliminating the excessive voltage amplitude of the surge;

In radio transmission systems, particularly in broadcasting, the modulation voltage is fed to the system through a balancedprogram line which feeds the modulation frequency ampli- 5 flers, The circuit from the line to the output of the amplifier is generally a balanced network using push-pull amplifying stages. Static disturbances or disturbances from any other source are likely to unbalance the program line, producing excessive voltage surges of short duration. These surges are transmitted in the amplifying system creating voltage peaks which may seriously affect the transmission circuit. Surge voltages may reach such magnitudes that component elements of the transmission system often burn out resulting in interruptions. The transmitted signal is also likely to bemarred by the static surges. interference due to static is often created at the transmitter and is picked up from the signal transmission. In other words, the static impulses are part of the broadcast transmission.

Devices heretofore used are effective only within a certain voltage range, for example, spark gaps or glow tubes. In certain transmission circults, the normal operating signal potentials are of sumciently high value to initiate discharge in glow-discharge type protective devices. In the circuit in accordance with this invention,

vacuum tubes are utilized as protective devices in'such manner that the anode-cathode impe'dance of the tubes forms a short circuiting path across the transmission circuit for voltages appearlng in excess to the normal signal potentials in the system. The control elements of the vacuumtub'es have an initial bias whereby conductance is prevented at the desired value of signal potentials and is effected only when this value is exceeded, and the bias is neutralized by an unbalanced voltage on the transmission line.

' Referring to the drawing, in Figure 1 a portion of a program transmission circuit is shown including a balanced input line connected to terminals I and 2 feeding into the primary winding 8 of the transformer 4 connects in the conventional arrangement of a push-pull input circuit to the grids 9 and Ill of vacuum tubes II and I2,

respectively.' The input circuit returns to the In radio receiver's the loud transformer I.

' winding 24 of thetransformer 2i is connected by means of conductors 26 and 21 directly. to the terminals of the primary winding 26 of the output transformer 23. Both the secondary-and primary windings above referred to form a balanced circuit in that the center taps are con-' nected to ground as shown. Effectively in shunt with the conductors 26 and 21 are vacuum tubes 30 and 3|, the anode 32 of tube 30 being connected to the conductor 26 and the anode 33 of tube 3| to the conductor 21. and 35 of the tubes 36 and 3| respectively, connect to the center tap of the winding 28 and also to ground. The grid circuit of the tubes 30 and 3| includes the interconnected grids 31 and 33 and returns to the grounded cathodes 34 and g 33 through the bias source represented here by the battery 43 and "the resistor 4| in series there' with. The latter forms the load resistance for the rectifier tubes '42 and 43. The anodes 44 and 45 of the tubes 42 and 43 respectively. connect to the terminals of the secondary winding 46 of the transformer I in a conventional full-wave rectifying arrangement in that the center tap connects to ground. -The interconnected cathodes .4'| and 48 return to ground through. the load resistor 4| which is bypassed by the condenser 49.

The transmission circuit just described was chosen as an example to illustrate the application. of the control circuit in accordance with this invention. Other types of balanced transmission circuits may as well be used. In certain applications the amplifier stage com rising the tubes ii and i2 may form any type of time delay The cathodes 34 resistance of the tubes.

network such as high pass, low pass or band/pass all represented by batteries. It is well known in the art that tubes of this type require a heatin: current which may be derived from an alternating-current source and that any suitable source of operating potentials may be used. The invention resides'in the application of vacuum tubes as variable impedance devices in shunt with a transmission circuit controlled from a potential source which derives energy from currents due to unbalance in the transmission line.

In describing the operation of the system, any unbalance in volta e on the program line feeding the terminals i and 2 will produce a current throu h the rimary winding 6 energizing the The voltage induced thereby across the secondary winding is rectifiedby the tubes 42 and 43 and appears as a charge across the condenser 46. When this voltage reaches a magnitude equal to or greater than the bias voltage of the battery 46, it will efiecti'vely neutralize the bias of the grids 31 and 3. and the tubes 30 and 3| will draw plate current. The change of bias initiating current conductivity of the tubes in effect is equivalent to a variable impedance shunting the conductors 26 and 21 to ground and thereby prevents excessive amplitudes of signal voltages in the transmission circuit. Since the control of the ellective anode-cathode resistance of the tubes 30 and 3| depends on the potentials derived from the unbalanced condition, it will be clear that the greater the voltage due to unbalance, proportionately lower will be the effective In other words, an increase of unbalance will inversely decrease the shunting resistance effectedby the tubes. At the same time due to the negative bias provided by the source 40, the control can never lower the transmission circuit voltage beyond the normal predetermined signal level, unless there is an unbalance in the transmission line feeding the circuit. This can be seen with reference to Figs. 2 and 3.

In Fig. 2, the signal voltage appearing across the transmission line terminals and 2 is shown with respect to time as one coordinate, and maximum permissible signal level is indicated between zero and A and B. Curve C shows the variations of signal voltage under normal transmission, the amplitude changes representing a characteristic variation of amplitude due' to speech or music. A sudden large peak amplitude exceeding the limits between A and B is shown at point D. This sudden surge may be due to static or other conditions unbalancing the input line. In Fig. 2, the illustration shows the effect of such surge voltages without the control system in accordance with this invention. Fig. 3 shows the signal voltage appearing across conductors 26 and 21 in a magnified form due .to the insertion of the amplifier stage in the circuit. The function of the tubes 30 and 3| in suppressing the excessive peak amplitude shows the flattening of the peak away below the maximum permissible signal amplitude values A and B. The dotted lines above that indicate the surge voltage which would appear across the lines 26 and 21 without tubes 30 and 3| in the circuit. The flattening 01'. the peak to nearly zero level occurred when the surge voltage appearing as a direct-current potential across the resistor 4| neutralized the bias from the source 46 to such an extent that the resultant low interval resistance of the anode cathode path of the tubes 30 and 3i had a shunting eiIect which decreased the amplitude of the signal voltage. Beyond the predetermined level between A and B, further increase of surge voltage is suppressed by the proportionate decrease of resistance of the tubes 30 and 3| effected by the positive control potential developed across the resistor 4| which increasingly biases the grids 31 and 36 as the surge peak increases merely further lowering the 'eifective shunt resistance.

I claim as my invention:- I 1. In a circuit, a balanced input line, a transmission channel including an impedancenetwork coupled to said line, an output circuit for said network, a pair of variable impedance elements line, a transmission channel including an impedance' network coupled to said line, anpoutput circuit for said network, a pair of vacuum tubeseach having anode, cathode and control elements.

shunting said network, the eflective impedance of riving said potentials from current flow due to unbalance in said line, and means for applying said potentials to said control elements at-such polarity as to vary the effective impedance thereof inversely with the magnitude of said potentials.

3. In a transmission circuit, a balanced input line, a transmission channel including an amplifier coupled to said line, an output circuit ior said amplifier, a pair or variable impedance eleof said elements being variable upon applicationoi bias-potentials to said control elements, means including a circuit common to said balanced input line and a rectifier in said circuit for deriving a bias potential from current flow due to unbalancein said line, and means for applying said potential to said control elements at a polarity I causing variation of the impedance thereof inments shunting said output circuit, the effective versely with the magnitude of said potential.

5. In a transmission circuit, a balanced input line, a transmission channel including an amplifier, an output circuit for said amplifier, a pair of vacuum tubes having anode, cathode and control elements shunting said output circuits, the shunting impedance of said vacuum tubes being variable upon application of bias potential to said control elements, a fixed source of bias potential maintaining the shunting impedance of said tubes at a value ineffective at normal signal transmission, means for deriving a bias potential from current flow due to imbalance in said line and means for applying said derived bias potential in series with and in opposition to said fixed source for varying the shunting impedance of said tubes inversely with the magnitude of said derived potential.

JOHN R. BOYmN. 

